This invention relates in general to motor controllers, and more particularly, to a high voltage bridge interface for AC (alternating current) and brushless DC (direct current) motor control partitioned for providing unilateral isolation between the control circuits and the high voltage power supply lines while improving the noise immunity and simplifying the total system integration.
Electric motors are common place in virtually all facets of the commercial and private sectors of modern society. The typical electric motor includes stationary stator windings magnetically coupled to the rotor windings which are an integral part of the rotating internal assembly. A motor control circuit regulates the current flow into the stator windings thereby controlling the energy transfer through the magnetic flux linkages to the rotor windings inducing current to flow through the latter and creating torque against the mechanical load. In brushless DC motors, for example, the stator windings are typically energized in a conventional three-phase manner to maintain a more uniform magnetic field about the rotor. The motor control circuit may include a power MOSFET (metal oxide semiconductor with field effect transistors) bridge having three pairs of transistors serially coupled between first and second power supply conductors operating at a predetermined DC value, i.e., 300 VDC, and ground potential, respectively, and a control circuit, such as Motorola, Inc. part number MC33034, for providing six output signals to turn the power MOSFETs on and off in an alternating manner and conduct the three-phase stator currents, as is understood. Although the gate terminals of the bottom-side power MOSFETs (ones having their source terminals coupled to the second power supply conductor) are often driven directly by the control circuit, the top-side power MOSFETs (ones having their drain terminals coupled to the first power supply conductor) require three turn-on/turn-off drive circuits coupled between the control circuit and the respective gate terminals thereof because of the full scale voltage swing between the first and second power supply conductors developed at the source terminals of the top-side power MOSFETs. In this configuration, the bottom-side MOSFETs are pulse width modulated (PWM) via the control circuit for controlling the rotational speed within the on-period while the top-side MOSFETs are commutated to steer the stator currents. In addition, the drive circuit uses a charge pump referenced to the first power supply conductor to raise the gate potentials of the top-side power MOSFETs above the source potentials of the same in order to achieve conduction therethrough.
The components of the conventional motor control circuit, i.e., charge pump and three drive circuits are complex and traditionally realized in discrete form principally for maintaining isolation between the control circuit and the high voltage power supply conductors. However, this topology is susceptible to noise because of the extreme voltage swing at the source terminals of the bottom-side power MOSFETs which operate at a higher PWM frequency whereby the top-side transistors may be rendered conductive or non-conductive at the inappropriate time. Hence, it would be desirable to pulse width modulate the top-side power MOSFETs and communtate the bottom-side power MOSFETs thereby improving the noise immunity of the motor control circuit and providing smoother motor starts under loaded conditions. In addition, since the commercial success of electric motors is often linked to the manufacturing and total system integration effort, it would also be desirable to reduce the cost associated therewith especially in high volume applications by partitioning the motor control circuit into a minimum number of integrated circuits (IC) while maintaining the high voltage isolation thereby avoiding the large number of discrete components prevalent in the prior art.
Hence, what is needed is an improved motor control circuit properly partitioned into a minimal number of ICs for providing unilateral isolation between the control circuit and the high voltage power supply lines while improving the noise immunity at the gate terminals of the power MOSFETs and simplifying the system integration.